1. Field of the Invention
The present invention relates to a drive control device for eliminating imbalance in main currents flowing across switching elements connected in parallel with one another, a module into which the switching element is integrated, and combined module having the switching elements connected in parallel which are the best suited for being applied to power conversion devices e.g. an inverter. In particular, the invention relates to an improvement of the same for precisely controlling the switching elements to eliminate the current imbalance.
2. Description of the Background Art
A power switching element e.g. a power IGBT, a power transistor, a power MOS transistor, and so on is widely utilized for a constituent element of a power conversion device e.g. an inverter, a chopper, and a converter. In an applied device represented by these power conversion devices, it is usual that plural power switching elements designed equally with one another are prepared and connected in parallel with one another especially when high rated current is required.
Main currents have to be kept equal with one another among the plural power switching elements connected in parallel because a burden is concentrated onto a particular power switching element and thereby a reliability of the applied device is degraded if the equality in the main currents fails. It is, however, usual that there are inequalities in electric characteristics among the plural power switching elements even if these elements are designed equally with one another. These inequalities arise from parasitic errors inevitably generated during manufacturing processes of the elements.
Inequality in saturation voltages across pairs of the main electrodes (e.g. collector-emitter saturation voltages V.sub.CE(sat) for IGBTs or bipolar transistors) is the most important of various inequalities among the plural power switching elements connected in parallel. Because the voltages across the respective pairs of the main electrodes of the elements connected in parallel are dominated by an element having the highest saturation voltage, and therefore, an element having lower saturation voltage shares larger current so as to raise up its saturation voltage.
Since the plural power switching elements connected in parallel usually share currents unequally as described above, an element across which larger current flows suffers higher current stress. As a result, a reliability of whole the elements connected in parallel or a reliability of the applied device is degraded.
The inequality in the main currents (i.e. current imbalance) is more serious problem for power switching elements having negative temperature coefficient. Because a larger main current flowing across the element having the negative temperature coefficient brings a rise in the temperature of the element so that the saturation voltage of the element decreases. The decrease in the saturation voltage causes the main current to flow further more largely. This vicious circle is repeated and thereby an excessive load is brought onto a particular element. As a result, the reliability of the applied device is degraded.
Two approaches are known as conventional techniques aiming at elimination of the inequality (i.e. imbalance) in the main currents respectively flowing across the power switching elements connected in parallel. The first one is a passive approach. In this approach, those power switching elements having identical or very similar electric characteristics are chosen as the elements to be connected in parallel. That is, power switching elements are subjected to such a screening as to minimize the deviations in characteristics thereof.
The second one is an active approach. In this approach, as disclosed in Japanese Patent Laying Open Gazette No. 8-213890, the main currents are so controlled by feedback control technique that inequality thereof is suppressed. More specifically, the main currents respectively flowing across the power switching elements are detected by current detection circuits and such control signals are transmitted to respective control electrodes of the power switching elements as to reduce the inequality in the detected main currents. This feedback control is performed by a control circuit formed of analogue circuit elements.
The first approach restricts the power switching elements to be employed for the device, i.e. produces those elements not to be used as the result of the screening. Accordingly, wastes are disadvantageously generated during the manufacturing processes of the elements. Further, the problem of the current imbalance is not completely solved even if the screening is carried out because it is usually not easy to find out power switching elements having identical electric characteristics.
On the other hand, the second approach performs the feedback control for equalizing the main currents; therefore, it does not require screening the power switching elements to make their electric characteristics uniform. However, since the control circuit for carrying out the feedback control is formed as an analogue circuit, it is disadvantageously hard to eliminate the current imbalance in high precision. Especially, the approach is disadvantageous in that it is hard to flexibly exclude the influence of the inequalities in various characteristics and thereby eliminate the current imbalance.
For example, inevitable inequality is usually generated during the manufacturing processes also in the characteristics of circuit elements forming the plural current detection circuits which are connected with the plural power switching elements respectively. This inequality gives rise to incorrect detection of inequality in the main currents of the power switching elements connected in parallel. It is hard for the analogue circuit to prevent the influence of the incorrect detection of the main currents. Therefore, the second approach in some cases happens to cause incorrect feedback control which generates the problem of inequality in the main currents rather than solves the same.
Further, in general there are inequalities not only in the characteristics of the current detection circuits but also in the transfer characteristics of the power switching elements themselves (e.g. respective relations between gate voltages V.sub.GE and collector currents I.sub.C in IGBTs), the characteristics of drive circuits of the power switching elements, the laying-out of wires which connect the plural power switching elements and the control circuit, the characteristics of circuit elements forming the control circuit, and so on. It is disadvantageously hard for the control circuit formed as an analogue circuit to implement a flexible and highly precise control which eliminates the current imbalance taking account of these inequalities as well.